1. Field of the Invention
This invention relates to an air/fuel ratio control system for an internal combustion engine, and more particularly to an air/fuel ratio control system for an internal combustion engine which uses a fuel containing alcohol and is equipped with the blowby gas ventilator.
2. Description of the Prior Art
There are in use engine air/fuel ratio control systems in which the amount of fuel supplied to the engine is optimized by regulating the fuel injection period of a fuel injection nozzle installed in the air intake passage. In these systems, the actual air/fuel ratio is calculated from the oxygen concentration of the engine exhaust gas, which is detected by, for example, an oxygen sensor installed in the engine exhaust passage, and the amount of fuel injected is increased/decreased so as to bring the actual fuel injection near the stoichiometric ratio. In this type of feedback control, so as to ensure that a problem such as failure of the oxygen sensor will not cause the amount of fuel injected to deviated greatly from the proper value, the range of adjustment is fixed in advance between a lower range set in the lean direction with respect to the air/fuel ratio and an upper limit set in the rich direction. And when the corrected amount calculated falls above or below this range, the calculated value is discarded and the upper or lower limit of the fixed range is used instead.
On the other hand, the fact that alcohol produces a smaller amount of nitrogen oxides and other harmful emissions than conventional fuels such as gasoline has led to increasing use of alcohol as an alternative fuel. While alcohol is sometimes used without blending, it is also often used as mixed with a conventional fuel in a blending ratio appropriate for the circumstances. One example is the hybrid fuel known as M85, which consists of 85% methanol (methyl alcohol) and the remainder of gasoline.
Alcohols not only have a large latent heat of vaporization but also have a smaller stoichiometric air/fuel ratio than gasoline (6.4:1 for methanol, for example, compared with 14.7:1 for gasoline) and thus have to be injected in large amounts. Just after starting, before the engine has warmed up, it is therefore possible that a part of the injected alcohol may not vaporize but leak into the crankcase through the gap between the piston and the cylinder wall. As a result, the alcohol gets into the engine oil and then, as the engine temperature increases, vaporizes and passes into the air intake system along with the blowby gas when the blowby gas ventilator is equipped.
The rate of vaporization of the alcohol from the engine oil is not constant but increases rapidly as the engine temperature approaches the boiling point of the alcohol (64.7.degree. C. for methanol). When this happens, the amount of alcohol recirculated to the intake system rises sharply, causing the air/fuel ratio to become rich. If feedback control is being conducted, the amount of fuel injected will therefore be corrected downward. However, the amount of this correction cannot exceed the lower limit of the fixed range mentioned earlier. Since this lower limit is not sufficient when the amount of vaporized alcohol being recirculated becomes large, engine emmission is impaired.
This invention was accomplished in view of the foregoing problem and has as its object to provide an air/fuel ratio control system for an internal combustion engine which ensures that engine emmission will not be impaired even when the amount of alcohol vapor recirculated together with the blowby gas becomes large.